The Allison in the M939 family of trucks is, like most all of them, pretty bullet proof within the performance envelope it was designed for and specified by the buyer for. But, no, an M939 series truck isn't an around the clock 65 MPH truck.
If I may interject a bit of reality; the difference in a 40 mile round trip between 55 MPH and 65 MPH is what, 4 minutes? What's the rush? There's not much in the work-a-day world worth going out of your way to be hurt for. If you have employees and operate equipment outside the design parameters you are asking to loose the farm.
You mention a manual transmission. It's not going to be faster. The HT-654 CR transmission in the M939 trucks has a lock-up torque converter in all five gears. Once it shifts and locks it has no more lost road speed than a manual transmission. The combination of engine max RPM, transmission final drive ratio and rear end gearing will determine the maximum sustained speed, not the transmission type.
When thinking of a working truck, the single biggest downside ,in my mind, of an M939 series truck, especially an A1 or A2 with the taller tires, is bed height. I'm 6'4" tall and the bed is at nearly shoulder height.
That said, the advantages of the M939 over the M44 (M35A2) series are real, and for a working farm truck those advantages seem decisive.
They have true spring air brakes as opposed to single circuit air assisted hydraulic brakes on the M44. With nearly all M44 series trucks if you loose any one hydraulic line you have NO brakes. With the M939 if you loose air pressure the spring brakes lock up and you stop.
The M939 has ABS; 'nuff said.
The M939 has power steering.
Remember that lock-up torque converter? The HT-454 CR has active transmission braking. When you foot is completely off the throttle it is actively braking (which led to the ABS upgrade but that's a different thread) and the transmission braking works even at low speeds and low RPMs. This, in turn, saves the service brakes for later and you really have to work at getting in a jam where the brakes can't stop you on even on a long steep slope etc. It's a nice safe bonus in a working truck, especially in hilly terrain.
The M939, if winch equipped, has the ability to run anything hydraulic that you brain cells can figure out how to plumb and control. Here's a thread with some details about adding a PTO to a w/o winch truck:
http://www.steelsoldiers.com/5-ton-up/89172-m939-m915a1-pto-info.html
Some years back there was a study of the comparison of driver fatigue, workload etc used by the TX DOT in a study as regarded future dump truck purchases. If my brain cell is correct the course was a 300 mile course that simulated 60% urban and 40% rural driving over varied terrain and compared a standard transmission (8 speed Road Ranger?) to an Allison automatic. The important part though is that the Allison driver shifted 8 times during the test. The standard transmission driver shifted over 2,400 times.
Operator comfort and fatigue counts for a lot, especially when a lot of low speed maneuvering is concerned. If all you need to do is pick the correct range on the transmission and drive you have a lot more attention to devote to safe and precise operation.
Buy the M939 truck and drive it as it was designed....
Lance
